This invention relates to a consumable electrode type d.c. arc welder and, especially, to an invertor controlled d.c. arc welder of consumable electrode type.
The invertor controlled d.c. arc welder is arranged to rectify an a.c. power from a commercial power supply by an input rectifier, convert its d.c. output into a high frequency a.c. power by an invertor composed of semiconductor switching elements, suitably reduce its voltage by an output transformer and again rectify it to obtain a d.c. power for welding. The welder of this type has such an advantage in that the device can be made small in size and weight since it needs no input transformer of large weight and the output transformer can be made small by increasing the output frequency of the invertor and, moreover, its output voltage can be effectively stabilized by controlling switching operation of the invertor using a difference between the output voltage and a predetermined reference voltage as its control voltage.
While d.c. arc welding of consumable electrode type, such as CO.sub.2 welding, MAG welding and MIG welding, is effected by supplying a stabilized d.c. power as above-mentioned from a welder to a welding load formed of a continuously fed welding electrode rod and a base material to be welded, separating the electrode rod shorted with the base material from the base material to form an arc therebetween and, thereafter, repeating shorting and arcing in similar fashion, undesirable spattering may take place if a current supplied to the welding load becomes excessive at the time of shorting. In order to avoid such spattering, it has been a general practice to provide a large d.c. reactor in the output side of the welder to prevent sudden increase of the load current. However, it has been a bar to reduction of the size and weight of the welder.
The inventors proposed means for preventing such undesirable spattering in the prior Japanese patent application No. H3-353356 (and its corresponding U.S. application Ser. No. 07/991,436, filed Dec. 16, 1992 now U.S. Pat. No. 5,272,314. More particularly, the shorting period Ts is detected from the output voltage V of the welder, which varies as shown in FIG. 1(a) and the above-mentioned reference voltage Sr is reduced by a predetermined value Sn during this period as shown in FIG. 1(b). Then, the above-mentioned invertor control voltage rises to reduce the output voltage during this period, thereby reducing the output current as shown in FIG. 1(c) to suppress spattering. However, it has been found that recovery of the output current is delayed at the early time of the arcing period as an aftereffect of reduction of the output current during the shorting period and less heat is supplied to the welding load. Thus, as shown in FIG. 2, a welded portion A rises abruptly from a base material B and, therefore, the welded region is narrowed to produce an insufficient weld having reduced mechanical strength.
Accordingly, an object of this invention is to provide an improved consumable electrode type d.c. arc welder which produces no defective weld as above-mentioned.